


Learn from the Mistakes I Have Already Made

by SaraStarchild



Category: Harry Potter - J. K. Rowling, Sherlock (TV)
Genre: Anniversary of Lily's Death, Detention, Flashbacks, Gryffindor John Watson, Gryffindor vs. Slytherin Rivalry, Gryffindor/Slytherin Inter-House Relationships, Johnlock feelings, M/M, Other, POV Severus Snape, Sherlock Being Sherlock, Sherlock Holmes Has Feelings, Sherlock is a Brat, Slytherin Sherlock, Snape is a nosy bastard, Snape is abusive, unorthodox teaching methods
Language: English
Status: Completed
Published: 2018-10-22
Updated: 2018-10-22
Packaged: 2019-08-05 19:47:14
Rating: General Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,542
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/16373918
Author URL: https://archiveofourown.org/users/SaraStarchild/pseuds/SaraStarchild
Summary: Professor Severus Snape has always hated Sherlock Holmes, but one day he snaps and gives Sherlock detention for talking back to him in class. He was originally going to have him write "I will not talk in class" a hundred times, but then, after coming to a startling realization about how Sherlock might feel about his Gryffindor best friend, John Watson, he thinks of a much better phrase to hammer into the young Slytherin's head...





	Learn from the Mistakes I Have Already Made

**Author's Note:**

> This has been a fic that I've wanted to write for a LONG time; today I finally sat down and wrote the damned thing. Enjoy!

“Fifty points from Gryffindor,” Professor Severus Snape deadpanned mid-sentence, still writing his current thought down on the board, and was met with the sound of groans and utterances of aggravation directed towards the Professor and the Gryffindor fifth-year who triggered the house point deduction: John Watson.

Granted, John Watson didn’t do anything to specifically _deserve_ the House Points taken away – most Gryffindors really didn’t when it came to Severus deducting points – but this time it was completely obvious that it wasn’t John’s fault.

Because, for the most part, it was his best friend’s fault.

His best friend being, the one and only, Sherlock Holmes.

Severus had hated Sherlock from day one; there was something about him, about his smile and his attitude and his blatant disregard of authority that really got under Severus’ skin. There were some days, in fact, that Severus felt like he was dealing with James Potter all over again, apart from a few differences. But the one difference that stood out the most, in Severus’ mind, was the fact that Sherlock was in Slytherin house, under Severus’ jurisdiction, which made the brat _his_ responsibility.  And, because of that, Sherlock had, over and over again, made Severus’ life more of a living hell than it already was.

It was getting toward the end of that day’s fifth year Gryffindor and Slytherin double Potions lesson that Severus made the point deduction, making the grand total eighty-five house points from Gryffindor, all of which were due to the two boys. The rest of the Gryffindors in the class groaned, hissing at Watson.

“John, what the hell?!”

“We’ll never win the house cup, at this point!”

_“Tell him to shut up!”_

“Why should I?” Sherlock muttered, eliminating the middle man. “I’d make a better teacher than –”

It’s then that Severus finally snaps, turning away from the board to glare down Sherlock Holmes. He could normally ignore the boy and just continue to make John Watson pay for it – he had gotten used to the quips and utterances over the past five years of teaching him – but it was the end of October – the anniversary of Lily Potter’s murder was fast approaching – and he couldn’t hold himself together any longer.

“Do you have something you’d like to _share_ with the class, _Mister Holmes?”_

Watson suddenly shot up straight from beside Sherlock, for if Sherlock had been caught, then John had been, too. That’s how it worked – Sherlock Holmes and John Watson; joined at the hip (or, at the very least, John would follow Sherlock around like a lost puppy with a sense of loyalty that made him better suited for Hufflepuff, but his nerve and awful choice of companionship landed him in Gryffindor) since John wandered into Sherlock’s otherwise-empty train compartment on their first trip to Hogwarts.

Just like with those two. James Potter and Sirius Black.

_“You’d better be in Slytherin,” he had told Lily Evans, but before she could answer she was cut off._

_“Slytherin? Who wants to be in Slytherin?” Potter asked, and then looked to Black. “I think I’d leave, wouldn’t you?”_

_“My whole family has been in Slytherin,” Black informed him._

_“Blimey, and I thought you seemed all right!”_

_“Maybe I’ll break the tradition,” Black said with a grin, and Severus had worried about what Lily thought of Slytherin now – now that these two boys had displayed their sheer dislike for the house Severus had been so keen on being a part of. “Where are you heading, if you’ve got the choice?” Black asked, and Potter lifted an invisible sword proudly._

_“‘Gryffindor, where dwell the brave at heart!’ Like my dad.”_

_Snape had scoffed, and James turned on him._

_“Got a problem with that?”_

_“No,” Severus had lied. “If you’d rather be brawny than brainy –”_

_“Where’re you hoping to go, seeing as you’re neither?” Black interjected, and it was when Potter roared with laughter that Lily finally decided she had had enough._

_“Come on, Severus, let’s find another compartment.”_

_“Oooooo...”_

_Potter had tried to trip Severus as he passed._

_“See ya, Snivellus!”_

Sherlock Holmes, on the other hand, leaned back, crossing his arms, watching Severus.

“Nope,” he replied, popping the ‘p’ as much as he could.

With another moment spent glaring at Sherlock, he turned back around and lifted his piece of chalk again, poised to write on the board. He was opening his mouth again to speak, just about to make the first sound of continuing his lecture, when Sherlock Holmes spoke again.

“Except that you’re wrong.”

And the piece of chalk snapped under the pressure of Severus pressing it against the chalkboard, and the only sound in the room was the broken piece falling to the stone floor, shattering into smaller pieces.

The class was dead silent, but there seemed to be a small, collective gasp as Severus spun around – all scared of what he’d do next.

All except for Sherlock Holmes, who sneered.

 _“What_. Was _that,_ Mister Holmes?” Severus asked, between gritted teeth.

Sherlock gestured to the chalkboard.

“You’re wrong,” he replied easily.

They watched each other, Severus’ eye twitching in rage.

“Care to _explain_ , Mister Holmes?”

“Gladly,” Sherlock said, standing, making his way to the shelf of ingredients. “You see, you just said that, in order to make the Draught of Peace, the syrup of hellebore needs to be aged for five to ten years before use. But, I’ve done some experimenting on my own, and I found out this interesting tidbit –” he said, pulling down a jar of the syrup, unscrewing the lid and taking a whiff. “– this’ll work perfectly – so I noticed that, when syrup of hellebore that has been aged five or six years, like this one, reacts to the moonstone powder –” he went on, taking a jar of moonstone powder and dumping it out into two separate piles on his desk, then pouring the syrup onto one of the piles of powder, mixing it together slightly with his spoon, “– the powder kind of clumps onto the syrup –”

“That’s what _simmering_ the potion is for, Mister Holmes, so now that you’ve embarrassed yourself enough, clean up this mess and –”

“Well, that’s what I thought too, at first, but just wait,” Sherlock continued, returning the first jar of syrup to the shelf in exchange for another, opening the jar and smelling the contents again before returning to his desk. _“This_ syrup of hellebore has been aged eight years, and, when _it_ reacts to the moonstone powder –” he then poured the syrup onto the second pile of powder, mixing that part with John’s spoon (which he gladly provided to Sherlock without a word), “– the powder mixes with the syrup smoothly, without the clumping. When making the potion _this_ way, the simmering process is almost cut in half, and it also creates a more potent potion. Sure, the potion still does it job with syrup that’s been aged five or six years, and I would recommend it for someone just starting out on taking the potion regularly, but if you are taking it regularly for help with anxiety or a similar disorder, syrup aged seven to eight years is perfect. On the other hand, if you’re having a panic attack, depending on the severity, drinking a Draught of Peace with syrup of hellebore aged nine or ten years will give instantaneous effects. Also, for shits and giggles, I brewed a Draught of Peace with syrup aged _eleven_ years, and found that it affects the drinker the same way Muggle anesthetic does, but without putting the drinker to sleep. So, ‘Professor,’ I would revise your lesson notes – and, by extension, my peers should change their class notes.”

Not even a Draught of Peace with syrup of hellebore aged _twelve_ years could calm Severus Snape down, especially with the way Sherlock was watching him, smirk on his face, waiting for Severus to tell him that he was right.

And he _was_ right – of course he was right. The boy could’ve been in Ravenclaw for his genius.

But he wasn’t going to admit that.

“Detention,” Severus decided, his tone so cold it seemed to freeze Sherlock, for a second. “Here. Tonight. And since you’re so concerned with Muggle practices, you can clean up the mess you just made on your desk by hand, and you can compare it to the cleaning job you would’ve had if you could’ve used your wand.”

Most of the Slytherins and some of the pureblooded Gryffindors in the class snickered at the jab against Muggles, but Sherlock stood frozen to the spot for a fraction of a second, entirely undetectable by anyone who couldn’t see his face, and then Sherlock snapped back into the present, making his way to the sinks, and Severus turned around, continuing his lesson as Sherlock wet a handful of paper towels.

When Sherlock returned to his desk to wipe away the mess, John Watson leaned forward, muttering to Sherlock’s downcast eyes.

“That was brilliant –”

“Do I need to take _more_ points from Gryffindor, Mister Watson?” Snape asked, still facing the board, and he could practically hear John sitting up straight again.

“No, Sir.”

“Then I do _not_ want to hear another _word_ out of _either of you_ for the rest of the _lesson,_ or you will be _joining_ Mister Holmes in _detention_ , Mister Watson. _”_ Severus ordered.

And he didn’t – he didn’t hear a single peep out of John or Sherlock, which was strange, considering how Severus could’ve taken a thousand house points from John Watson and Sherlock would still keep opening his mouth. But the thought of John getting into trouble – tangible, very real trouble – seemed to stop Sherlock up short. He knew that Watson came from a pure-blood family – his little sister was also in Gryffindor, and had a tendency of tattling on her older brother to their mother. Maybe their mother had threatened John, in some way, if John got himself into trouble once more? Maybe Sherlock was trying to prevent that, in any way he could? That at least showed Severus that Holmes, a boy who didn’t seem to legitimately care about anything, cared about John…

It showed that he cared, or something more –

And, with a near startling realization, Severus suddenly had an idea.

* * *

 When Sherlock walked into the Potions classroom that night, Severus was ready for him.

“Sit,” he ordered, skipping all forms of pleasantries, gesturing lazily to Sherlock’s usual seat, the desk he had been sitting at just a few hours ago, where Severus had already laid a few feet of parchment out for him, with a quill and ink bottle beside it.

Sherlock sat down at the desk, glancing back and forth between Severus and the parchment.

“You will be writing lines,” Severus announced.

“Obviously –” Sherlock started, but that was all he was able to get out before Severus flicked his wand toward the chalkboard, flipping the board over, revealing the phrase of which Sherlock had to write:

_I will not fall in love with a Gryffindor._

“What –?” Sherlock murmured, reading the words a few more times, as if he hadn’t read them correctly the first time.

“I see the way you look at John Watson –”

“John –?” Sherlock repeated, his pale cheeks reddening, and Severus could barely contain the sneer that he so wanted to give him.

“You need to be taught a lesson, Mister Holmes. This wasn’t the lesson I had expected to teach, tonight, but it will be the one that you learn. One-hundred lines.”

“This is brainwash –” Sherlock argued.

“Two-hundred.”

“Increase the numbers all you want; it still doesn’t guarantee that I’ll learn it –”

“Three hundred! And next time you open your mouth it’ll be five hundred, and I’ll send an owl home to that Muggle brother of yours!” Sherlock stopped short, his face paling. Severus sneered. “Remind me, what was his name, again? Mycroft?”

If Severus was a different teacher, like his old Potions professor, Horace Slughorn, he might’ve taken the opportunity to ask what a Mudblood like him was doing in a house like Slytherin. But Severus knew for a fact that not all Slytherins were purebloods.

Sherlock gaped at Severus for a few moments more clamping his mouth closed.

“That’s better,” Severus said. “Now get to work.”

It took a grand total of two hours for Sherlock to complete his lines, and by the end of it, Severus could practically feel Sherlock’s hand cramping from across the room. But, as soon as he was finished, Sherlock rose from his chair, swiping up the parchment, and roughly placing it on Severus’ desk, on top of the parchments that Severus had been working on while waiting for Sherlock to finish.

Severus glanced over Sherlock’s work.

“I should have you rewrite this; your handwriting’s atrocious,” Severus mused, then looked up at Sherlock. “But I believe you got the message.” He waited for a moment for Sherlock to answer, to agree or deny or to say anything at all, but when Severus decided that Sherlock wasn’t going to, he continued on, “I imagine that was more than difficult for you to do, Mister Holmes, but it’s for your own good, trust me. Even if John Watson was a person who had an attraction to the same gender, he would never reciprocate your feelings.”

Sherlock’s eyes narrowed, just a fraction, as he eyes scanned over his face, and Severus thought that Sherlock was preparing a rebuttal – that he hadn’t learned, after all – but then his eyes widened, as if shocked.

“Oh my god – _Lily Potter?”_ Sherlock gasped, and, within the blink of an eye, Severus pulled out his wand, standing from his desk.

_“Obliviate!”_

_“Protego!”_ Sherlock countered, somehow pulling out his wand even faster, blocking the spell. Severus opened his mouth to cast the spell again, but Sherlock was ready for it. _“Expelliarmus!”_ he exclaimed, and Severus’ wand flew from his hand, clattering onto the stone floor.

They stood for a moment, staring at each other, Severus with eyes narrowed, livid, and Sherlock with widened eyes, full of…was that _fear?_ Sherlock kept his wand trained on Severus, ready to protect himself, if he needed to, and Severus knew that yes, it was indeed fear in the boy’s eyes.

“Get out,” Severus ordered, finally, fighting to keep his voice level. “And don’t you _dare_ utter that name _ever_ again – not to _anyone_ – or I swear to _God_ I will –”

He was cut off by the sound of the door slamming behind Sherlock.

Severus then glanced down at the desk, at the scroll that Sherlock had just turned in, reading the words that were written upon it as if for the first time.

_I will not fall in love with a Gryffindor._

_I will not fall in love with a Gryffindor._

_I will not fall in love with a Gryffindor._

It really was for his own good, Severus thought, a single tear escaping from his eye and rolling down his cheek. As much as he hated the boy, and as much as he saw James Potter when he looked at him, he also saw a fraction of himself. A broken heart was a broken heart, and if Severus could teach Sherlock Holmes to avoid it, he would.

Because it was too late for Severus Snape.

It was far too late.

**Author's Note:**

> The flashback is copied word-by-word from Harry Potter and the Deathly Hallows - I do not own any of it, it all belongs to JK Rowling. Snape and her characters and the location and the Houses and the potion ingredients are hers, too.  
> And Sherlock and John belong to Sir Arthur Conan Doyle and also the BBC.  
> Honestly, if you think about it, none of this is mine, because even the words I use were once coined by somebody else...  
> Well, the thought of the concept for the story, so...that's mine?  
> It's 3:00 in the morning don't @ me.


End file.
